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Question about magnetic induction

  1. May 8, 2012 #1
    hi everyone iam happy to join physics forums
    i 'd like to ask a question about magnetic induction
    if i have a current carrying wire(connected to a battery=V) of length (L)& of intensity (I) & perpendicular to a uniform magnetic field(B) then a force act on it (F=LIB)
    so when it moves....if it changes the no. of magnetic field lines inside its circuit then it induces emf also a current which is oppostie to the one already passing in the circuit so the (F) decreases gradually until the induced emf become equal to the emf of the battery
    so the final situation
    Wire with no current (because of equilibrium that happened) moving with constant velocity.....

    is that right?......or i have missed something......
  2. jcsd
  3. May 8, 2012 #2


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    welcome to pf!

    hi max11011! welcome to pf! :smile:
    but why would it change the number of field lines (the magnetic flux)?

    it's not rotating, and the field is uniform, so won't the lines cut ("cookie-cutter" style) stay the same? :confused:
  4. May 9, 2012 #3
    Max: Yopu have picked up on 2 effects here
    1) When a current carrying wire is placed in a magnetic field it experiences a force (BIL)
    this is the principle behind the electric motor.
    2) When a conductor moves through a magnetic field an emf is induced that opposes the change producing it. In an electric motor this is known as a 'back emf'
    In an ideal case, with no energy losses, the motor will reach a speed where the back emf = the applied emf.
    This does not mean the current will be zero !!!!! If there are no energy losses it is possible to have a current with no resultant emf !!!
  5. May 10, 2012 #4
    Thanks. ..tiny-tim for ur reply
    but Although the field is uniform and no rotation...,the area of the circuit increases gradually so does the magnetic lines in the curcuit consequently.....a current is indced.
    The picture may make it more clear.

    Thanks. ..truesearch...for ur reply...
    so if the motor or the current carrying wire isnt ideal the back emf wouldnt equal forward emf.

    Attached Files:

  6. May 10, 2012 #5


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    hi max11011! :wink:
    ah, yes, if the area is increasing :smile:

    (btw, shouldn't "dots" be coming up towards us? :wink:)
  7. May 10, 2012 #6
    If there is resistance in the circuit then the back emf will equal the supply emf -Ir.
    This is the simplest case to consider when there is resistance present
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